FAN4800 Low Start-Up Current PFC/PWM Controller Combos
© 2005 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN4800 Rev. 1.0.5 9
Functional Description
The FAN4800 consists of an average current controlled,
continuous boost Power Factor Correction (PFC) front
end and a synchronized Pulse Width Modulator (PWM)
back end. The PWM can be used in either current or
voltage mode. In voltage mode, feed forward from the
PFC output bus can be used to improve the PWM’s line
regulation. In either mode, the PWM stage uses conven-
tional trailing-edge, duty-cycle modulation. This patented
leading/trailing edge modulation results in a higher
usable PFC error amplifier bandwidth and can signifi-
cantly reduce the size of the PFC DC bus capacitor.
The synchronization of the PWM with the PFC simplifies
the PWM compensation due to the controlled ripple on
the PFC output capacitor (the PWM input capacitor). The
PWM section of the FAN4800 runs at the same fre-
quency as the PFC.
In addition to power factor correction, a number of pro-
tection features are built into the FAN4800. These
include soft-start, PFC over-voltage protection, peak cur-
rent limiting, brownout protection, duty cycle limiting, and
under-voltage lockout (UVLO).
Power Factor Correction
Power Factor Correction treats a nonlinear load like a
resistive load to the AC line. For a resistor, the current
drawn from the line is in phase with and proportional to
the line voltage, so the power factor is unity (one). A
common class of nonlinear load is the input of most
power supplies, which use a bridge rectifier and capaci-
tive input filter fed from the line.
The peak charging effect, which occurs on the input filter
capacitor in these supplies, causes brief, high-amplitude
pulses of current to flow from the power line, rather than
a sinusoidal current in phase with the line voltage. Such
supplies present a power factor to the line of less than
one (i.e., they cause significant current harmonics of the
power line frequency to appear at the input). If the input
current drawn by such a supply (or any nonlinear load)
can be made to follow the input voltage in instantaneous
amplitude, it appears resistive to the supply.
To hold the input current draw of a device drawing power
from the AC line in phase with and proportional to the
input voltage, that device must be prevented from load-
ing the line except in proportion to the instantaneous line
voltage. To accomplish this, the PFC section of the
FAN4800 uses a boost mode DC-DC converter. The
input to the converter is the full-wave, rectified, AC line
voltage. No bulk filtering is applied following the bridge
rectifier, so the input voltage to the boost converter
ranges (at twice line the frequency) from zero volts to a
peak value of the AC input and back to zero. By forcing
the boost converter to meet two simultaneous conditions,
it is possible to ensure that the current drawn from the
power line is proportional to the input line voltage.
One of these conditions is that the output voltage of the
boost converter must be set higher than the peak value
of the line voltage. A commonly used value is 385V
DC
, to
allow for a high line of 270V
AC
rms. The second condition
is that the current drawn from the line at any given
instant must be proportional to the line voltage. Estab-
lishing a suitable voltage control loop for the converter,
which in turn drives a current error amplifier and switch-
ing output driver, satisfies the first of these requirements.
The second requirement is met by using the rectified AC
line voltage to modulate the output of the voltage control
loop. Such modulation causes the current error amplifier
to command a power stage current that varies directly
with the input voltage. To prevent ripple, which necessar-
ily appears at the output of boost circuit (typically about
10V
AC
on a 385V
DC
level), from introducing distortion
back through the voltage error amplifier, the bandwidth of
the voltage loop is deliberately kept low. A final refine-
ment is to adjust the overall gain of the PFC section to be
proportional to 1/V
IN
2
, which linearizes the transfer func-
tion of the system as the AC input voltage.
Since the boost converter in the FAN4800 PFC is current
averaging, no slope compensation is required.
1. PFC Section
1.1 Gain Modulator
Figure 1 shows a block diagram of the PFC section of
the FAN4800. The gain modulator is the heart of the
PFC, as the circuit block controls the response of the
current loop to line voltage waveform and frequency,
RMS line voltage, and PFC output voltages. There are
three inputs to the gain modulator:
1. A current representing the instantaneous input voltage
(amplitude and wave shape) to the PFC. The rectified
AC input sine wave is converted to a proportional cur-
rent via a resistor and is then fed into the gain modula-
tor at I
AC
. Sampling current in this way minimizes
ground noise, required in high-power, switching-power
conversion environments. The gain modulator
responds linearly to this current.
2. A voltage proportional to the long-term RMS AC line
voltage, derived from the rectified line voltage after
scaling and filtering. This signal is presented to the
gain modulator at V
RMS
. The output of the gain modu-
lator is inversely proportional to V
RMS
2
(except at
unusually low values of V
RMS
, where special gain con-
touring takes over to limit power dissipation of the cir-
cuit components under heavy brownout conditions).
The relationship between V
RMS
and gain is called K
and is illustrated in Figure 5.
